1. Field of the Invention
The present invention relates to a photoelectric conversion device and an image sensing system.
2. Description of the Related Art
A photoelectric conversion device including an operational amplifier is available as an active-type photoelectric conversion device. As shown in FIG. 9, a photoelectric conversion device 100 includes a pixel array, a plurality of readout circuits 130, a vertical shift register 123, and a horizontal shift register 119. A plurality of pixels GU11 to GUmn (m: natural number, n: natural number) are two-dimensionally (in the row and column directions) arranged in the pixel array. The readout circuit 130 is arranged for each of the columns in the pixel array (the plurality of pixels GU11 to GUmn). Each of the vertical shift register 123 and the horizontal shift register 119 is connected to the pixels GU11 to GUmn.
The vertical shift register 123 selects a readout row (pixel row) by activating a select signal PSEL1, PSEL2, . . . to turn on a select transistor 105. In each pixel on the selected row, an amplification transistor 104 converts, into a signal (a noise signal or a photogenerated signal), a charge signal read out from a photodiode 101 to a floating diffusion (to be referred to as an FD hereinafter) via a transfer transistor 102 according to an activated transfer signal PTX1, PTX2, . . . . The amplification transistor 104 outputs the signal (the noise signal or the photogenerated signal) via the select transistor 104 to a vertical signal line 106. The readout circuit 130 reads out the converted signal via the vertical signal line 106 for each column of the pixels, and stores it. The horizontal shift register 119 sequentially turns on horizontal transfer switches 114 according to horizontal shift signals H1, H2, . . . , and sequentially outputs the signals held in the readout circuit 130 for each column, via a horizontal signal line 116 and an output circuit 118. Note that, by means of activating a reset signal PRES1, PRES2, . . . , a reset transistor 103 in each pixel turns on to reset the FD.
In the readout circuit 130, a clamp capacitance 108 stores the signals read out via the vertical signal line 106. An operational amplifier 120 amplifies the difference according to a capacitance ratio of a capacitance 121 to the clamp capacitance 108 between the stored noise signal and photogenerated signal based on a reference voltage VREF input from an external power source. A line memory 112 holds the amplified signal when a transistor 110 turns on in response to activation of a signal PT. Note that, by means of activating a signal PCVR, a transistor 107 turns on to reset the vertical signal line 106, and that, by activating a signal PCOR, a transistor 109 turns on to reset the operational amplifier 120.
Japanese Patent Laid-Open No. 2005-269471 proposes a technique which uses a readout circuit including an operational amplifier which amplifies a stored signal based on the reference voltage (clamp voltage Vclp) input from the external power source, as described above.
However, according to the technique disclosed in Japanese Patent Laid-Open No. 2005-269471, when disturbance noise is mixed into the reference voltage to be input to the operational amplifier in the readout circuit, the disturbance noise may be superposed on a signal (a noise signal or a photogenerated signal). Accordingly, random stripe noise sometimes appears in a formed image in accordance with the difference between the noise signal and the photogenerated signal.